In recent years, a reduction in weight and thickness has been strictly required for batteries as a power supply for portable devices and the like. Thus, in regard to materials used to cover the batteries, laminate films capable of a further reduction in weight and thickness have been used more and more instead of conventional metal cans in which there is a limit to weight and thickness reductions. This laminate film which can be freely formed into different shapes, compared with a metal can, comprises a thin metal film or a laminate of a thin metal film and a heat-seal resin film.
As a representative example of a laminate film used for materials used to cover the batteries, there is a three-layer laminate film that has a heat-seal resin film, which is a heat seal layer, laminated on one side of an aluminum thin film, which is a thin metal film, and a protection film laminated on the other side.
Generally, in a film-covered battery which uses a laminate film for a covering material, a battery element comprising a positive electrode, a negative electrode, an electrolyte and the like is hermetically sealed (hereinafter simply called “sealed”) by covering the battery elements with the covering material in such a manner that heat-seal resin film is positioned inside, and by heat-sealing the covering material around the battery element. Polyethylene or polypropylene, for example, is used for the heat-seal resin film, while a nylon film or a polyethylene terephthalate film, for example, is used for the protection film.
For reference, as a battery element, other than chemical batteries such as a lithium battery, a nickel hydrogen battery and the like, those having a charge accumulating function such as a capacitor also use laminate film as a covering material.
In a film-covered battery, lead terminals are connected to a positive electrode and a negative electrode of a battery element, respectively, for leading the positive electrode and negative electrode to the outside of the covering material to extend these lead terminals from the covering material. The lead terminals are connected to the battery element by ultrasonic welding or the like before the battery element is sealed. Also, the battery element is sealed by sandwiching the battery element with two sheets of covering material, and by heat-sealing the covering materials along the peripheral edge. The covering materials are heat-sealed by first heat-sealing three sides of the covering materials for formation into a bag. Subsequently, air is exhausted from the interior of the covering materials to produce a vacuum inside the covering materials, the covering materials are brought into close contact with the battery element by atmospheric pressure, and the one side that remains is heat-sealed in this state.
When the battery element has a certain degree of thickness, the general approach is to form one of the covering materials into the shape of a collared container shape by deep-draw molding, such that the battery element is readily received therein, cover the covering material formed into the container shape from above the battery element, and join the collar by thermal fusing.
For preventing external moisture and the like from being introduced into the film-covered battery and to prevent the electrolytic solution and the like within the film-covered battery from escaping to the outside, a thin metal film of aluminum or the like is used in the covering film as a barrier layer, but the heat-seal resin film layer is exposed along the edge of the joint section around the battery element, causing a leak path based on the molecular transport phenomenon of the resin itself. Therefore, to improve the reliability of sealing for the film-covered battery, the leak can be reduced by increasing the width of the joint section to extend the transmission path and to increase the resistance, however this causes the problem that a larger projection area of the film-covered battery. Thus, JP-A-2002-25514 proposes a reduction in projection area by folding a joint section toward a battery element receiving section.